1. Field of the Invention
The present invention concerns space switching of optical signals, as used in for example in a fiber optic telecommunication network.
2. Description of the Prior Art
The nodes of these networks include space switching devices comprising passive distributors and active selectors. One to n passive distributors each supplies on one output only a fraction 1/n of the power of the signal received at its input. Also, each active selector introduces losses. For this reason a device of this type conventionally includes doped fiber amplifiers. This type of amplifier includes a doped optical fiber for guiding optical signals to be transmitted. It also includes pump means injecting pump light into the fiber in order to amplify the signals and thereby compensate the power losses mentioned above. The present invention is more generally concerned with the situation in which the amplifier device is not necessarily an optical fiber but is pumped optically.
For example, another major problem in switching matrices using these selectors is crosstalk between active selectors. An active selector can switch signals by connecting an optical waveguide, referred to hereinafter as the "common guide" with respect to this selector and these signals, to an individual guide selected from a plurality of individual guides that can be connected to the common guide. It transmits signals from the individual guides to the common guide or from the common guide to the individual guide. For example, the crosstalk can be expressed in a simplified manner by a coefficient r such that, if power WA is received on the common guide and transmitted by the selected guide, unwanted power r.WA is transmitted by each of the other individual guides or, if power WB is received at an unselected individual guide, unwanted power r.WB is transmitted by the common guide.
A first prior art switching matrix includes arrangements to reduce the crosstalk coefficient.
It is described in the article: PHOTONIC SWITCHING II, PROCEEDINGS OF THE INTERNATIONAL TOPICAL MEETING, vol. 29, 12 Apr. 1990-14 Apr. 1990 KOBE, JAPAN, pages 122-125, XP 000333141 Y. SATO ET AL "An Erbium-Doped Fiber Active Switch Controled by Coded Pump Light". It includes a plurality of doped fiber amplifiers distributed over various guided paths that the signals to be switched can take. A plurality of switchable feed waveguides convey respective pump waves to the amplifiers in order to energize them, i.e. to effect the optical pumping of their doped fibers.
An optical decoder receives this wave on its input side to transmit it to the waveguides. In accordance with coding of the wave, it feeds selectively at all times those amplifiers on the path selected at that time. As a result the signals on the selected paths are amplified and the other signals are absorbed by the amplifiers that are not energized.
This first prior art matrix is complex.
An object of the present invention is to provide a simple optical switching matrix having a low crosstalk coefficient.